First, I am so sorry for your loss.
Second, So much more information is needed to even attempt to know what went wrong. We have to be careful not to jump to conclusions way to quickly and make assumptions that make no logical sense.
We all need to do research on the effects that inbreeding has on our animals. It can very well affect all animals if in fact they are all from the same family. Heart failure is very common in inbred gliders. As well as kidney and liver failure and neurological problems. But without the necropsy you will never know. And even that needs to be done right away.
You need the background of every glider to even start to figure out the cause. With no pedigree and no background, the glider should ever be bred. This is the results when we try to breed cheap. Gliders that are 'rescues', should remain 'rescues'. Poor breeding result in gliders dying and hearts being broken.
Here are some things you should know. Keep records of each, vet will need also.
#1. Where did they come from? 'Honest' Breeder? or Pocket Pets = Mall, Craigslist, a friend, classifieds?
#2. Were the 12 gliders properly introduced? Such as: were they taken to vet first for clean bill of health, quarantined for 30 days then retested?
#3. What supplements were used? Were they supplemented properly and regularly? Gliders will die quicker, w/o proper supplements.
#4. Were you properly educated before you started collecting them? Did you have any help with caring for them? Or with the cost of vetting all twelve?
#5. What diet were you using? Healthy nutritional diet? or the instant stuff?
#6. How long since you added the last one to colony?
#7. Are all 12 gliders in the same cage?
Oh, and the honey would not have saved them. It would have only given them a fast sugar boost(which would have speed up the heart more). But I know you were panicking to save them. I think it was just out of your hands -or even a vets hands- in this case. Again, I'm so sorry.